We have purified to homogeneity two distinct forms of human interleukin 1 (IL 1 alpha 18K Da, pI 5; and IL 1 beta - 18K Da, pI 7) from myelomonocytic cell line (THP-1) culture supernatants and cell extracts. This directly confirmed the existance of at least two distinct IL 1 gene products based on cDNA cloning of IL 1 by other groups. Using radiolabeled purified human IL 1 beta, the IL 1 receptor on Epstein Barr virus transformed human B lymphocytes (EBV-B) was studied. The receptor number detected on cells of IL 1-responsive EBV-B lymphocyte lines was small ranging from 100-200 receptor/cell with a Kd of about 1 to 5 10-10M. Furthermore, we showed that IL 1 alpha and IL 1 beta bind to the same receptor. Since every human EBV-B cell line so far tested by us constitutively produces low levels of IL 1 alpha, the low level of IL 1 receptor expression on EBV-B could be due to down-regulation of IL 1 receptor expression by autologously produced IL 1 alpha. This possibility was suggested by the observation of down-regulation of IL 1 receptor by IL 1 itself on a human large granular lymphocyte cell line (YT) that did not produce its own IL 1 and expressed approximately 7,000 receptors/cell with a Kd- about 1 x 10-10M. The mechanism of production of IL 1 was also investigated. Study of post-translational modification of human IL 1 showed that precursor intracellular IL 1 alpha in LPS stimulated human monocytes is phosphorylated at serine residue. Since both IL 1 alpha and beta precursors are located in the cytosol, evidence of selective phosphorylation of precursor human IL 1 alpha may suggest that IL 1 has another unknown role inside the cell. We have reported that IL 1 is cytostatic/cytocidal for several tumor cell lines, including a human melanoma cell line, A375. A specific receptor for IL 1 was detected only on the IL 1 sensitive clone (about 500 receptors/cell, Kd = 1 x 10-10M). Study of the mechanism of antiproliferation activity by IL 1 on A375 cells showed that ornithine decarboxylase is causally involved. These biochemical studies of production, biochemical properties, receptors and mechanisms of action of IL 1 should enable us to utilize IL 1 more effectively as a BRM.